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Recovery from acidification of lochs in Galloway,south-west Scotland, UK: 1979-1998

R. C. Ferrier, R. C. Helliwell, B. J. Cosby, A. Jenkins, R. F. Wright

To cite this version:R. C. Ferrier, R. C. Helliwell, B. J. Cosby, A. Jenkins, R. F. Wright. Recovery from acidificationof lochs in Galloway, south-west Scotland, UK: 1979-1998. Hydrology and Earth System SciencesDiscussions, European Geosciences Union, 2001, 5 (3), pp.421-432. �hal-00304622�

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Recovery from acidification of lochs in Galloway, wouth-west Scotland, UK : 1979–1998

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Hydrology and Earth System Sciences, 5(3), 421–431 (2001) © EGS

Recovery from acidification of lochs in Galloway, south-westScotland, UK: 1979-1998R.C. Ferrier1, R.C. Helliwell1, B.J. Cosby2, A. Jenkins3 and R.F. Wright4

1 Macaulay Institute, Aberdeen, Scotland, AB15 8QH, UK2 Department of Environmental Science, University of Virginia, Charlottesville, VA 22901,USA3 Centre for Ecology and Hydrology, Wallingford, Oxon OX10 8BB, UK4 Norwegian Institute for Water Research, PO Box 173, Kjelsås, N-0411 Oslo, Norway

Email for corresponding author: [email protected]

AbstractThe Galloway region of south-west Scotland has historically been subject to long-term deposition of acidic precipitation which has resultedin acidification of soils and surface waters and subsequent damage to aquatic ecology. Since the end of the 1970s, however, acidic depositionhas decreased substantially. The general pattern is for a rapid decline in non-marine sulphate in rainwater over the period 1978-1988 followedby stable concentrations to the mid-1990s. Concentrations of nitrate and ammonium in deposition have remained constant between 1980 and1998. Seven water quality surveys of 48 lochs in the Galloway region have been conducted between 1979 and 1998. During the first 10 years,from 1979, there was a major decline in regional sulphate concentrations in the lochs, which was expected to have produced a decline in basecations and an increase in the acid neutralising capacity. But sea-salt levels (as indicated by chloride concentrations) were approximately25% higher in 1988 than in 1979 and thus short-term acidification due to sea-salts offset much of the long-term recovery trend expected in thelochs. During the next 10 years, however, the chloride concentrations returned to 1979 levels and the lochs showed large increases in acidneutralising capacity despite little change in sulphate concentrations. From the observed decline in sulphate deposition and concentrations ofsulphate in the lochs, it appears that approximately 75% of the possible improvement in acid neutralising capacity has already occurred overthe 20-year period (1979-1998). The role of acid deposition as a driving factor for change in water chemistry in the Galloway lochs isconfounded by concurrent changes in other driving variables, most notably, factors related to episodic and year-to-year variations in climate.In addition to inputs of sea-salts, climate probably also influences other chemical signals such as peaks in regional nitrate concentrations andthe sharp increase in dissolved organic carbon during the 1990s.

Keywords: acidification, recovery, Galloway, sulphur, nitrogen

IntroductionAcidification of surface waters in the UK uplands is linkedto the emission and subsequent deposition of oxides ofsulphur (S) and nitrogen (N) from the atmosphere (Battarbeeet al., 1990). Reconstruction of historical surface water pH,through the analysis of diatoms in lake sediment cores,shows that acidification generally occurred in the mid tolate nineteenth century (DOE, 1995). The extent ofacidification is determined by the sensitivity of the lakerelative to the amount of acidity deposited. This sensitivitycan also be related to the available pool of base cations inthe catchment soils and the weathering rate of the underlyinggeology. In these respects, the lochs of the Galloway regionof south-west Scotland represent some of the most severelyacidified waters in the UK with a combination of slow

weathering granite bedrock, thin organic soils and a highdeposition flux of S and N.

The first synoptic regional survey of water chemistry inthe UK was carried out in Galloway in 1979 (Wright andHenriksen, 1980). The results from this survey indicate thatlochs situated on the granite intrusion are extremely acidicrelative to those on the surrounding sedimentary bedrockreceiving similar deposition loads. A repeat survey of thesame lochs in 1988 showed a 42% decrease in sulphate (SO4)concentrations across the region resulting in a substantialincrease in acid neutralising capacity (ANC) and pH in themost acidic lochs (Wright et al., 1994). This was consistentwith a decrease in S deposition over the same period. Afurther five water chemistry surveys were undertaken in theperiod 1988-1998 and analysis of the chemical changes from

R.C. Ferrier, R.C. Helliwell, B.J. Cosby, A. Jenkins and R.F. Wright

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these more recent surveys are assessed in relation to theearlier observations. This represents a unique summary ofthe water chemistry of the Galloway lochs from 1979-1998.The aims of this paper, therefore, are to determine themagnitude of these recent chemical changes, identify themajor external drivers of change and to discuss what furtherpotential recovery is to be expected.

Regional descriptionThe Galloway region is a classic acid sensitive environmentwith highly siliceous granitic bedrock covered by thin,patchy, organic rich and generally acidic soils, which offersonly limited ability to neutralise acid inputs from theatmosphere (Wright et al., 1994). Decades of acid depositionand, more recently, large-scale afforestation in the region,have exacerbated the problem of soil and water acidification(Rees and Ribbens, 1995). The Galloway region alsoreceives some of the highest rainfall in Scotland with annualaverage exceeding 2000 mm.

The bedrock geology of the Galloway area is characterisedby Paleozoic (mainly Silurian) sedimentary rocks (shale,slate, greywacke, sandstone and conglomerate) into whichgranitic plutons are intruded (Daysh, 1974). The soildistribution is closely related to altitude and slope with large

areas of brown forest soils occurring below 250–300 m onvery steep land and peaty podzols at higher elevations oron less steep slopes. Poorly developed alpine rankers andlithosols occur in high altitude areas (> 600 m), particularlyin the Merrick mountains. Peat, often deep (> 50cm), iswidely developed in this area.

To the south and west of the region, moorlandcommunities including Juncus effusus-Sphagnum recurvum,Carex nigra and Erico-Sphagnetum papillosi are dominantin areas of dystrophic peat, peaty gleys and low base statusnoncalcareous gley soils (Birse and Robertson, 1976). Inthe 1920s, the Forestry Commission began purchasing landin the area for afforestation. Nearly 70 000 ha waspurchased, of which approximately half had been plantedby 1974 (Edlin, 1974). The Forestry Commission landincludes much of the higher-elevation, less accessible hillcountry, areas that are of relatively low value for agricultureand sheep grazing. Planting included not only the nativeScots pine but also a number of exotic species such asEuropean larch, Douglas fir and Norway and Sitka spruces.

The catchments included in these surveys were selectedto represent a regional distribution of acidified lochs inrelation to acid deposition, forest cover, soil type andgeology (Table 1 and Fig. 1).

Fig. 1. Location of the study lochs in south west Scotland


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Sampling and analysisSurface water sampling began in 1979 and included 48 lochsin the vicinity of the three largest granitic plutons in theGalloway region (Fig. 1). These lochs were resampled inMarch/April of 1988, 1993, 1994, 1996, 1997 and 1998.Dip samples were collected either from the loch directlyabove the outflow stream or from the loch shore at about50 cm water depth where no loch outflow existed. Sampleswere taken and stored in polyethylene bottles at 4ºC in thedark and were filtered through 0.45 µm membrane filtersprior to analysis. Samples were analysed for a full suite ofchemical determinands by standard procedures (Patrick etal., 1991) at the Freshwater Fisheries Laboratory, Pitlochryand the Norwegian Institute for Water Research, Oslo, forthe 1979 and 1988 loch surveys. For subsequent surveysthe analyses were conducted at the Macaulay Institute,Aberdeen. Strict quality control procedures between thelaboratories ensured that there were no systematicdifferences in analytical results. pH was measuredpotentiometrically; calcium (Ca), magnesium (Mg), sodium(Na) and potassium (K) by atomic adsorptionspectrophotometry; nitrate (NO3), chloride (Cl), and SO4

by ion chromatography; aluminium (Al) by automatedcolorimetry; dissolved organic carbon (DOC) by total carbonanalyser (oxidation to carbon dioxide (CO2) with detectionby IR). ANC was calculated as ∑ base cations (Ca, Mg, Na,K) minus ∑ acid anions (SO4, NO3, Cl).

ResultsDEPOSITION

An estimate of the changes in wet deposition chemistry tothe Galloway region has been obtained from the Loch Deecollector (Grid Reference NX 468 779) established in 1982as part of the UK Acid Deposition Monitoring Network(UKADMN). A longer term perspective has been obtainedfrom wet deposition data at the Eskdalemuir collector (GridReference NT 234 028), also as part of the UKADMN, withrecords dating back to 1973. This collector is locatedapproximately 80 km to the east and 25 km to the north ofthe Loch Dee site.

The deposition data from the Eskdalemuir collectorfollows two different protocols; mean annual concentrationsare calculated from both mean daily concentrations andmean weekly concentrations, the latter forming the basis ofthe UKADMN data (RGAR, 1983, 1987, 1993, 1997).Comparison of the wet deposited non-marine SO4

concentration from the two methods at Eskdalemuir (Fig.2) shows no difference. Within the paper the non-marinecomponent of sulphate (nmSO4) was determined by:

nmSO4 = SO4 – (R* Cl)

where nmSO4 is the non-marine concentration of SO4, andR is the ratio of that ion to Cl in seawater (0.104 for SO4

where concentrations are in µeq l-1). The Loch Dee data isbased on mean weekly concentrations and demonstratesgreater annual variability in concentrations but the sameoverall trend as at Eskdalemuir. The general pattern is for arapid decline in nmSO4 over the period 1978-1988approximating to a > 50% reduction, followed by morestable concentrations during the 1980s and early 1990s, anda subsequent reduction (10% of 1978 values) until 1998.Concentrations of NO3 and ammonium (NH4) at both

Table 1. Catchment characteristics of 48 lochs in theGalloway region

Attributes Number oflochs

Geology Granite 14Greywackes 34

Soils (dominant) Peat 5Peaty gley 9Peaty podzol 17Sub-alpine podzols 8Brown Forest Soils 9

Land Use Forested (1988) 28Moorland (1988) 20

Loch:catchment < 0.1 33area ratio 0.1- 0.2 10

> 0.2 5

Fig. 2. Wet deposition of non-marine SO4 at Eskdalemuir andLoch Dee


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Eskdalemuir and Loch Dee have remained constant between1980 and 1998 (Fig. 3).

Similarly, concentrations of Ca and Mg have remainedrelatively low and constant between 1980 and 1998 (Fig.4). Chloride concentrations, however, have been extremelyvariable at Loch Dee reflecting the proximity of this site tothe coast.

concentration of each ion (in µeql-1) by the number of yearsin the interval. These slopes are then sorted from lowest tohighest and plotted against the % rank of each value. Theresulting ranked-slope plots can be used to determine thepercentage of sites that had slopes greater or less than someparticular value.

Concentrations of SO4 in all of the 48 lochs declinedsharply during the first 10 years 1979-1988 (Figs. 5 and 6).Median change for the 48 lochs was –6 µeq l-1 per year(Table 2). During the next 10 years, 1988-1998, however,SO4 concentrations remained relatively constant with aboutequal numbers of lochs showing positive and negativechanges (Fig. 7). This pattern of SO4 concentrations in thelochs closely reflects the trend in SO4 deposition in theregion; the major decline in SO4 deposition occurred duringthe early 1980s (Fig. 2).

Concentrations of NO3 in the lochs, on the other hand,show no systematic regional pattern during the 20 years(Fig. 5). Concentrations were somewhat lower in 1988relative to 1979, but then increased again during the period1988-1998 (Figs. 6 and 7). In addition, deposition ofinorganic N in the region has not changed systematicallyover this period. Samples collected in spring 1996 hadexceptionally high concentrations of NO3 (Fig. 5).`Concentrations of sea-salt-derived ions, particularly Cl, inthe lochs varied greatly, but did not show any major trendover time for the region (Fig. 5). Levels were relatively lowin the first survey, higher in 1988 and then lower againduring the late 1990s (Table 2). Linear changes are thus

Fig. 3. Concentrations of NO3 and NH4 in wet deposition atEskdalemuir and Loch Dee

Fig. 4. Base cation and Cl concentrations in wet deposition atLoch Dee, 1980-1998

HYDROCHEMISTRY

The distribution of changes in the concentration of each ionbetween each of the surveys can be illustrated with ranked-slope plots for each interval between surveys (1979-1988,1988-1998 and 1979-1998). The change in each ion for eachinterval is expressed as a rate of change for the interval inquestion by dividing the observed change in the

Table 2. Median regional concentration of major ions in1998 and slopes of changes for the periods 1979-1988, 1988-1998 and 1979-1998. All data as ìeq l-1, all changes presentedas ìeq l-1 per year.

Median Change Change Changeregional 1979– 1988– 1979–chemistry 1988 1998 19981998

Ca 49.9 -1.35 0.34 -0.24Mg 49.9 -0.73 -0.07 -0.11Na 171.9 3.43 -1.83 1.26K 7.6 -0.23 0.10 -0.04NO3 8.8 -0.76 0.68 0.07SO4 59.1 -5.78 -0.39 -2.41Cl 180.2 5.64 -5.50 0.22SBC 281.3 0.50 -1.42 1.07SAA 248.1 -0.74 -6.01 -1.97ANC 33.2 1.18 4.50 3.26


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Fig. 5. Box and whisker plots of selected determinands (SO4, NO3, C, ANC, pH and DOC) for the seven regional surveys in Galloway, since1979. All data as µeq l-1 except pH, and DOC (mg l-1). The box represents the 25 and 75 percentile and the whisker is the 10th and 90th

percentile, outliers are also shown.


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Fig. 6. Percentage of sites showing positive or negative trends for water quality variables in the period 1979-1988. All data as µeq l-1


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Fig. 7. Percentage of sites showing positive or negative trends for water quality variables in the period 1988-1998. All data as µeq l-1.


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positive in the first 10 years (Fig. 6) and negative duringthe second 10 years (Fig. 7). In this instance, no significantregional trend was observed during the 20 year record (Fig.8).

As a consequence of the changes in SO4, NO3 and Cl, themedian sum of strong acid anions (SAA) decreased by about1 µeq l-1 per year over the period 1979 to 1988; the largedecrease in SO4 was offset by an increase in Cl. Over thesecond 10 year period (1988-1998), the median SAAdecreased further by 6 µeq l-1 per year, largely due to lowerCl concentrations (Table 2). For the entire 20 years themedian change in SAA was –2 µeq l-1. All but four of thelochs showed negative change in SAA over the entire period(Fig. 8).

Charge balance necessitates that decreases in SAA arecompensated by either a decrease in base cations or anincrease in ANC. Median change in sum of base cationswas +1 µeq l-1 per year over the entire 20 years (Table 2).Trend analyses of Ca and SO4 are very similar. Loch datafrom the first 10 years showed a decline in Ca (median slopeof –1.4 µeq l-1 per year), with little change during the next10 years. Trends in Mg and K are similar to those for Ca.

During the first and second 10 year period, increases inANC were concurrent with decreases in SAAconcentrations, whereby the median slope of ANC in thefirst 10 years equalled +1 µeq l-1 per year and +4 µeq l-1 peryear during the second 10 year period (Figs. 6, 7, and 8;Table 2). The increases in ANC were due to decreasedconcentrations of hydrogen (H) ion and Al. Dissolvedorganic carbon (DOC) appears to have increased from 1994to 1998.

DiscussionMonitoring of surface water chemistry began in earnest inthe UK in the late 1970s following the recognition thatsurface water acidification presented a problem. Theapproach to monitoring has taken two directions: systematicand routine sampling of individual sites at monthly intervals(or less) and synoptic surveys sampling surface standingwaters in a region on an annual frequency (or more).Determination of chemical trends from the former reliesupon detecting a signal from noise introduced by the short-term chemical response to rainfall input and seasonality. Inthe latter, the signal must be detected from noise introducedby different physical characteristics of a large populationof lakes. In practice, a balance between both approachesprovides the most robust results and interpretation.

The data presented come from a regional survey of 48lochs conducted seven times during a 20-year period. Thegeneral pattern of change identified by these spatially

extensive but temporally sparse data, mirrors changes inindividual lochs sampled more frequently over the same timeperiod by Harriman et al. (1995, 2001) and NEGTAP (2001).Temporal changes in loch chemistry identified in this study,therefore, provide a mechanism to examine the regionalresponse of both highly acidified as well as more moderatelyacidified waters. For example, the data of Harriman et al.(2001) for Loch Enoch show large decreases in SO4

concentrations during the 1980s with smaller changes duringthe 1990s, which are concomitant with deposition trendshighlighted in Fig 2. This pattern of change is alsoaccompanied by small decreases in concentrations of basecations and an increase in ANC (Harriman et al., 2001).The seven regional surveys conducted over 20 years,superimposed on detailed chemistry from high-frequencymonitoring at selected sites, thus appear to captureadequately the major trends in water chemistry in Gallowaylochs.

Surface waters in the Galloway region are stronglyinfluenced by inputs of sea-salts from the atmosphere. Thisis generally the case over large areas of the UK (Monteithand Evans, 2000). Inputs of sea-salts occur in conjunctionwith storms and since Cl is very mobile in soils, Clconcentrations in streams and lochs show large variationover time. The cations in the sea-salts, mainly Na and Mg,are not as mobile in soils as Cl since both cations participatein cation exchange reactions. In acidified soils, such as thosetypical of the Galloway region, the incoming Na and Mgexchange in part for inorganic Al and H, resulting indecreased ANC of the run-off. This episodic acidificationdue to sea-salt deposition has been documented in studiesfrom Loch Dee in Galloway (Langan, 1987), Norway(Hindar et al., 1994) and Maine, USA, (Heath et al., 1992)and was demonstrated experimentally at Sogndal, Norway(Wright et al., 1988).

Episodic input of sea-salts must be considered in casethey obscure ongoing trends of acidification, as is apparentlythe case in this 20-year record from the Galloway lochs. Itis especially important, therefore, to ensure that lowfrequency regional survey data is consistent with highfrequency data collected from sites in the same region. Themajor decline in SO4 concentrations in the lochs during thefirst ten years should have produced declines in base cationsand increases in ANC. But the sea-salt levels (as shown byCl concentrations) were about 25% higher in 1988 than in1979 and thus short-term acidification due to sea-salts offsetmuch of the long-term recovery expected in the lochs.During the next 10 years, however, Cl concentrationsdecreased back to 1979 levels and the lochs show largeincreases in ANC despite little change in SO4 concentrations.The trend data indicate that year-to-year variations in sea-


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Fig. 8. Percentage of sites showing positive or negative trends for water quality variables in the period 1979-1998. All data as µeq l-1.


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salt deposition caused a 5-10 year delay in the recovery inANC in the lochs. This is a similar result to that of Monteithand Evans (2000) in their analysis of a 10-year record ofwater chemistry trends in 22 lakes and streams in the UKAcid Waters Monitoring Network (AWMN). They showedthat variable inputs of sea-salts causes ‘noise’ in theconcentrations of major ions such that long-term trends maybe partly masked.

Nitrate concentrations in streamwaters have been shownto be related to the amount of N deposition with sitesreceiving between 10 and 25 kg N ha-1 yr-1 showing a variableresponse, such that some sites show elevated levels, whilstothers do not. Sites with deposition inputs above 25 kgN ha-1, however, have shown elevated NO3 outputs (Diseand Wright, 1995). For Galloway, present day depositionof N is higher than the 25 kg N ha-1 threshold and someevidence of N enrichment and potential elevated N releaseshould be expected. The current 20-year records suggestthat there has been no regional increase in N saturation, atleast under the criterion of increased leaching of inorganicN from the soil (Aber et al., 1989). The lochs and theircatchments retain 80–100% (Helliwell et al., 2001) ofinorganic N deposition and have done so for the 20-yearperiod. The number of lochs with increasing NO3

concentrations is equal to those with decreasing trends overthe 20 year period. If N saturation is occurring, the rate ofbreakthrough is very slow (i.e. very long time lag). Helliwellet al. (2001), however, highlight that N leaching was evidentfrom forested catchments in Galloway with soil C/N ratiosbelow 20, in line with that predicted for European forestsoils (Gundersen, 1995; Gundersen et al., 1998).

The exceptionally high NO3 concentrations in 1996 areapparently a larger regional phenomenon, as such a peak isalso reported from the 10-year records of the AWMN(Monteith et al., 2000). The cause of this peak is probablyrelated to climate and Monteith et al. (2000) suggest thatunusually cold winters give rise to higher NO3

concentrations in surface waters in the spring.From the decline in SO4 deposition and concentrations of

SO4 in the lochs, it appears that about 75% of the possibleimprovement in surface water quality has already occurredover the 20-year period 1979–1998. The median non-marineSO4 concentration in the lochs declined by 67 µeq l-1 fromabout 108 µeq l-1 in 1979 and to about 41µeq l-1 in 1998.The marine fraction of SO4 amounts to about 20 µeq l-1.Median non-marine SBC declined by about 23 µeq l-1 fromabout 115 to 92 µeq l-1, while median ANC increased byabout 40 µeq l-1 from about 3 to about 43 µeq l-1. The declinein SO4, therefore, was compensated one third by decreasedSBC and two thirds by increased ANC. From these changes,a rough estimate of the maximum future recovery in ANC

can be obtained. If in the future non-marine SO4 depositionwere to decrease to zero, the concentrations in the lochswould decrease by a further 20 µeq l-1 to median value ofabout 20 µeq l-1 (an estimate of median background non-marine SO4 concentration). This would be compensated byabout a 7 µeq l-1 decrease in SBC and a 13 µeq l-1 increasein ANC. This rough estimate indicates, therefore, that ofthe total improvement in ANC (median change of53 µeq l-1), about 75% (median change 1979 to 199840 µeq l-1) has already occurred.

This estimate assumes no long-term changes in NO3

concentrations. Nitrate in the future could go either up,down, or stay the same. Continued inputs of inorganic Nfrom deposition should, over the long term, result in Nsaturation with increased concentrations of NO3 in run-off(Aber et al., 1989). The 20-year record from Galloway lochsindicates, however, that this process is very slow, as therehas been no significant increase in NO3 concentrations inthe lochs over the period 1979–98 despite 20 years of highN deposition in the area. On the other hand, NO3

concentrations in the future could decrease due to decreaseddeposition of inorganic N. If the emission reductionrequirements of the Gothenburg protocol are indeedimplemented, then N deposition in Europe should decreaseby over 50% by the year 2010 relative to the base year 1990(expected reductions: SO4 63%; NOx 41%; NHy 17%). Butin this case, since median NO3 concentration in 1998 wasonly about 10 µeq l-1, future decreases in concentrationscould only cause a few µeq l-1 increase in ANC. The potentialimpact of land use and management on terrestrial N cycling,in particular commercial afforestation, is also unclear(Ferrier et al., 1995).

The increase in DOC concentrations during the 1990s inthe Galloway lochs is part of a general trend reported frommany areas of northwestern Europe. The 10-year recordfrom the UKAWMN shows striking increases in DOC inmost sites (Monteith and Evans, 2000), as do similar datafrom waters in Norway (Skjelkvåle et al., 1998). Here, again,climate is probably the driving factor as the increases haveoccurred simultaneously in waters over a very large areaand in both acidified and non-acidified systems (Monteithand Evans, 2000). Increased DOC will result in increasedorganic anion availability which will buffer pH and increaseANC. Thus, the pH levels in the lochs in Galloway haverecovered less than expected from the increase in ANC.

In summary, it is clear that the lochs in Galloway haveshown substantial recovery in response to a long-termdecline in the anthropogenic deposition of primarily S and,to a lesser degree, N. The regional pattern of recovery isconsistent with that observed for specific lochs where highfrequency monitoring has been carried out. The role of acid


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deposition as a driving factor for changes in water chemistryin the Galloway lochs, however, is confounded byconcurrent changes in other driving variables, most notablyfactors related to variations in climate. Sea-salt depositionand other climatic influences, are regionally important inthis area of Scotland and strongly influence the observedhydrochemical response, especially N and C dynamics.

AcknowledgementsWe are indebted to the numerous colleagues who have beeninvolved in the regional water surveys since 1979. Thisproject was supported financially by the Commission ofEuropean Communities projects DYNAMO (ENV4-CT95-0030) and RECOVER:2010 (ENK1-CT-1999-00018), theScottish Executive Environment and Rural AffairsDepartment (SEERAD), the UK Natural EnvironmentResearch Council and the Norwegian Institute for WaterResearch. Analytical support from the Freshwater FisheriesLaboratory (Pitlochry), the Macaulay Institute and theNorwegian Institute for Water Research is gratefullyacknowledged.

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